1. Field of the Invention
The invention relates to circuit interrupters and more particularly to circuit interrupters including means for limiting short circuit current therethrough.
2. Description of the Prior Art
Circuit interrupters are widely used in industry to protect electrical apparatus from overcurrent conditions. They are designed to continuously carry current at a level necessary for normal operation of the apparatus being protected. At currents above this level, the circuit breaker will interrupt the flow of current through the electrical apparatus, thereby preventing damage which can result from such overcurrent operation. The maximum level of current which the circuit interrupter will carry without interrupting is known as the maximum continuous current. Beyond this level, interruption will occur within an interval determined by the amount by which the current exceeds the maximum continuous rating. However, there is a maximum interruptable current beyond which the circuit interrupter will be damaged and will not successfully interrupt the current flow. Some circuits are capable of supplying as much as 100,000 amperes or more. To provide a breaker alone having a maximum interruptable current of this order would require a breaker with massive components having a very high cost. It is, therefore, desirable to provide the circuit interrupter with means in addition to the circuit breaker which will operate upon the occurrence of heavy short circuit currents having values above the maximum interruptable current rating of the breaker along, thereby extending the maximum interruptable current of the entire circuit interrupter.
One way of providing the current limiting action is to connect a conventional fuse device in series circuit relationship with the circuit breaker. Such a fuse device is described in U.S. Pat. No. 3,593,251 issued to the applicant. A current interrupter including such as fuse device is described in U.S. Pat. No. 3,638,157 issued to Zelco J. Kruzic. Both of the above mentioned patents are assigned to the assignee of the instant application. Conventional fuse devices require replacement after each current-limiting interruption.
Re-healable fuses do not require replacement after each interruption. The life of such devices is finite, however, and they, too, eventually require replacement.
Another method for limiting current through a circuit interrupter under heavy short circuit current conditions is described in U.S. Pat. No. 3,136,921 issued to H. D. Dorfman et al. In that device, an additional arc is introduced in series circuit relationship with the circuit breaker contacts by an electromagnet also in series circuit relationship with the contacts. Another device in which current limiting is accomplished by an arc induced electro-dynamically is described in U.S. Pat. No. 3,555,471 issued to J. F. Mitskevich et al. Pat. No. 3,745,369 issued to Kiyoshi Yamagata also described a device employing electrodynamic means for current limiting in a circuit interrupter. Such devices require periodic cleaning, maintenance and eventual replacement.
It would be desirable to provide a circuit interrupter including self-restoring current-limiting means of simple low-cost construction. It would also be desirable to provide current-limiting means employing no moving parts which does not require maintenance or replacement following current-limiting circuit interruption as is required in current-limiting fuse devices.